Wireless multicast/broadcast is becoming increasingly important for efficient dissemination of streaming video as well as common data because the shared nature of wireless medium provides a natural support of wireless multicast and broadcast. However, one limitation of the wireless multicast is that the multicast transmission rate is limited by the mobile terminal with the lowest signal strength in the multicast group.
Beamforming antennas have been widely used to boost the received signal strength in unicast scenarios by concentrating the signal energy in a specific direction at a client. If beamforming can be applied to boost the received power at the weakest client of a multicast group, it can potentially improve the performance of wireless multicast significantly. Nevertheless, it is challenging to apply beamforming technologies to multicast/broadcast transmissions because there is an inherent tradeoff between multicasting and beamforming. While beamforming increases the signal energy on a particular direction, it also reduces the energy on other directions, thereby restricting the wireless broadcast advantage.
An attempt has been made to address the integrated issue of multicasting and beamforming by using a combination of an omni-directional transmission in a first step, followed by one or more multiple single-lobe beamforming transmission in the second step to cover the clients left behind from the omni-directional transmission.
There has been provided a rigorous formulation of the switched beamforming multicasting problem as dividing the beam patterns into multiple partition/group and then sequentially transmitting each group of beam patterns. The objective of this formulation is to minimize the total transmission delay. Two power allocation problems have also been considered: an Equal power EQP model, where the power is evenly distributed among different beams in a given group, and an Asymmetric Power ASP model, where the power distribution across different beams is asymmetric and can be optimally chosen to maximize the minimum SNR in each beam. Also considered were two cases in each model depending on whether the rate is a continuous or discrete function of the SNR values and provide processes for all cases. However, these prior attempts, dividing beam patterns into groups and determining the weight of each antenna for the transmission to the group, need improved efficiency and better performance techniques for dividing the user groups.
Accordingly, there is a need for improved multicasting with beamforming antennas.